I doubt that Nvidia will invest in mobile GPUs at this point. The Tegra never really caught on in mobile: the Switch is basically the only customer for it.
Nintendo will need to find another vendor for the next Switch (of which there are plenty in the mobile space).
Nvidia said they will be working with Nintendo for the next 20 years. So no they will be around for Switch 2 even if a custom SOC which is likely the case. You also want to keep OG Switch games fully BC.